WIP: mesa: Clean up nomenclature for pipeline stages.

Previously, we had an enum called gl_shader_type which represented
pipeline stages in the order they occur in the pipeline
(i.e. MESA_SHADER_VERTEX=0, MESA_SHADER_GEOMETRY=1, etc), and several
inconsistently named functions for converting between it and other
representations:

- _mesa_shader_type_to_string: gl_shader_type -> string
- _mesa_shader_type_to_index: GLenum (GL_*_SHADER) -> gl_shader_type
- _mesa_program_target_to_index: GLenum (GL_*_PROGRAM) -> gl_shader_type
- _mesa_shader_enum_to_string: GLenum (GL_*_{SHADER,PROGRAM}) -> string

This patch tries to clean things up so that we use more consistent
terminology: the enum is now called gl_shader_stage (to emphasize that
it is in the order of pipeline stages), and the conversion functions are:

- _mesa_shader_stage_to_string: gl_shader_stage -> string
- _mesa_shader_enum_to_shader_stage: GLenum (GL_*_SHADER) -> gl_shader_stage
- _mesa_program_enum_to_shader_stage: GLenum (GL_*_PROGRAM) -> gl_shader_type
- _mesa_progshader_enum_to_string: GLenum (GL_*_{SHADER,PROGRAM}) -> string

In addition, MESA_SHADER_TYPES has been renamed to MESA_SHADER_STAGES,
for consistency with the new name for the enum.
!UPSTREAM: mesa: Store gl_shader_stage enum in gl_shader objects.

This reduces confusion since gl_shader::Type is sometimes
GL_SHADER_PROGRAM_MESA but is more frequently
GL_SHADER_{VERTEX,GEOMETRY,FRAGMENT}.  It also has the advantage that
when switching on gl_shader::Stage, the compiler will alert if one of
the possible enum types is unhandled.  Finally, many functions in
src/glsl (especially those dealing with linking) already use
gl_shader_stage to represent pipeline stages; storing a
gl_shader_stage in the gl_shader object avoids the need for a
conversion.

TODO:
- Get rid of TypeFoo.
glsl: Change _mesa_glsl_parse_state ctor to use gl_shader_stage enum.
glsl: Make more use of gl_shader_stage enum in link_varyings.cpp.
Make more use of gl_shader_stage enum in lower_clip_distance.cpp.
glsl: Make more use of gl_shader_stage enum in ir_set_program_inouts.cpp.
Get rid of TypeFoo